Igniter plug with improved electrode

ABSTRACT

This invention relates to an igniter plug adapted for continuous sparking operation for extended periods of time at temperatures as high as about 2,000* F. wherein the shell body constituting the ground electrode is provided with a ductile iridium metal insert ring having high oxidation and spark erosion resistance, the insert being positioned in the spark gap area and being either electron or laser beam welded to the shell in order to produce a narrow weld zone of well-alloyed material with minimum recrystallization of the insert metal.

United States Patent Van Uum et al.

[451 Sept. 12, 1972 IGNITER PLUG WITH IMPROVED ELECTRODE Inventors: Donald R. Van Uum, Davison; John 1. Jalbing, Millington, both of Mich.

Assignee: General Motors Corporation,

Detroit, Mich.

Filed: Feb. 25, 1971 Appl. No.: 118,862

Field ofSearch ..3l3/1l8, 130, 131, 138, 139, 313/141, 142

Primary Examiner-Roy Lake Assistant Examiner-Darwin R. Hostetter Attomey-Sidney Carter and Peter A. Taucher [57] ABSTRACT This invention relates to an igniter plug adapted for continuous sparking operation for extended periods of time at temperatures as high as about 2,000 F. wherein the shell body constituting the ground electrode is provided with a ductile iridium metal insert ring having high oxidation and spark erosion resistance, the insert being positioned in the spark gap area and being either electron or laser beam welded to the shell in order to produce a narrow weld zone of well-alloyed material with minimum recrystallization of the insert metal.

3 Claims, 2 Drawing Figures I. 33:0, 0 2 H l j I /j 3??? IGNITER PLUG WITH IMPROVED ELECTRODE It is well known in the spark plug art that the operating life of a spark plug may be extended by using iridium electrodes, the iridium being recognized as having oxidation and spark erosion resistant properties. However, use of such material has not achieved general acceptance in the art, not only due to the fact of high cost but also because of the difficulty in obtaining a satisfactory bond between the base metal of the electrode and the iridium, the iridium portion being prone to break and drop off into the engine. Relevant prior art is believed to be U.S. Pat. No. 2,642,053 issued June 16, 1953 to A. G. Dowson, U.S. Pat. No. 3,146,370 issued Aug. 25, 1964 to R. J. Van Duyne et al. and U.S. Pat. No. 3,399,290 issued Aug. 27, 1968 to M. K. Pardell.

It is therefore an object of our invention to provide an iridium insert in the ground electrode portion of the spark gap of an igniter plug, the interface or weld zone between the shell and the insert being achieved by electron or laser beam welding to produce a narrow and well-alloyed zone with minimum recrystallization of the insert metal to form a strong bond.

It is another object of our invention to use a metal insert having a fibrous structure for maximum ductility and high resistance to fracture.

It is a still further object of our invention to provide for a weld zone which penetrates the full depth of the insert to assure good heat transfer to the shell body.

These and other objects of our invention will be apparent from the description which follows taken in conjunction with the drawing in which:

FIG. 1 is a partially sectioned view of an igniter plug embodying our invention and FIG. 2 is an enlarged broken-away sectional view of the shell tip to show the details of our invention.

As shown in FIG. 1, our invention is embodied in an igniter plug of the type commonly used in aircraft jet engines wherein the spark gap is provided for within the center portion of the plug at the tip end thereof normally positioned within the combustion can of the engine. It will also be apparent from the description which follows that our invention is equally applicable to plug designs wherein the spark gap is exterior of the plug at the end thereof. In view of the extremely high cost of aircraft and engines as well as the necessity for extremely high reliability for the entire vehicle assembly as well as for such critical components as the igniter plugs, the use of a high cost noble metal such as iridium is not unreasonable, provided the plug assembly can be made with the necessary reliability and assurance that the normally brittle metal can be securely bonded to the ground electrode. Another problem which our invention solves is that of achieving an igniter plug which lends itself to long periods of sustained operation without the necessity of replacement due to overheating of the sparking tip of the plug with resultant rapid wear due to oxidation and spark erosion. This is achieved by providing a weld zone which extends the full depth of the interface between the metal insert and the shell body and thus assures good heat conduction away from the spark gap insert to the shell body. Continuous operation over extended periods is desirous to assure continuous burning in the event of the occurrence of what would otherwise be a flame-out condition.

As shown in FIG. 1, igniter 1 comprises a shell body 2 which constitutes the ground electrode portion of the igniter and within which there is concentrically positioned in gas-tight relationship therewith an insulator body 5 having a centerbore 7 therethrough, the centerbore having positioned therein a center electrode 9. The shell body 2 may be formed of any suitable material such as a nickel steel, i.e., Inconel, a trademark for an alloy containing approximately 76 percent nickel, 16 percent chromium, and 6 percent iron, such materials having desirable high temperature oxidation resistance characteristics. As shown, the shell body 2 is also provided with a plurality of cooling air intake apertures 11 through the upper portion thereof, the cooling air flowing through the annular space between the shell body and the insulator and out through a plurality of holes 13 in the tip end of the shell body.

As is more clearly shown in FIG. 2, the firing end 15 of the center electrode 9 is formed of spark erosion resistant metal such as tungsten, the sparking end thereof being in abutment with the end of the insulator 5. The inner surface of the shell body 2 is formed at the firing end thereof with a annular shoulder 17 on which there is seated a metal insert 19 formed of ductile iridium. As is clear from FIG. 2, the firing end of the igniter plug is designed so as to provide a spark gap 21 formed by and across the end surface of the insulator 5 which abuts the firing end 15 of the center electrode and the surface of the metal insert 19 which is secured integrally with the shell body in such manner as to provide a good electrical and thermal interconnection.

In accordance with our invention, and based on tests, we have found that it is necessary to provide an interface between the shell 2 and the insert 19 which will securely bond the insert to the shell body without breaking or fracture under the high vibration levels and elevated temperatures of as high as about 2,000 F. encountered during normal operation. We have found that such an interface may be achieved when using such normally brittle metals as iridium by welding the insert to the shell using a high energy, narrow beam technique such as electron or laser beam welding. A narrow, high intensity beam is essential for forming a nonfrangible weld zone 23 in view of the fact that we have found that where a broad weld zone is created, as where brazing or other welding techniques are used, the large amount of recrystallization of the iridium results in a weak bond area readily susceptible to cracking with the result that the insert quickly breaks up and drops off into the engine. Maintaining a narrow weld zone such as shown at 23 in FIG. 2 minimizes the degree of recrystallization while forming a well-alloyed mixture of shell and insert metals and assures a tough bond or strong interface with the shell body. Similarly, the high intensity narrow welding beam assures the penetration of the weld through the full depth of the electrode to produce an interface having optimum thermal conducting capability between the electrode and the shell.

We have also found it advantageous in forming the insert 19 to use metal wire which has been hot-worked, as by drawing the wire through a draw-die, in order to impart ductility to the wire by forming a fibrous crystal structure instead of the face centered cubic crystal structure normal to iridium. It should also be noted in this regard that the use of a high energy, narrow welding beam to produce a narrow weld zone tends to preserve the fibrous structure of the insert with resultant greater ductility and resistance to fracturing.

The table below shows the variation in sparking voltage of the igniter plug of our invention over a 24 hour period when using an iridium insert.

Sparking Voltage Kilo-Volts The tests are conducted using a 4 joule exciter for sparking the plug with the lower portion of the igniter maintained at a l,000 F. and at 100 psi pressure using ambient shop air. Thebreakdown or sparking voltages were checked at room temperature and at the indicated pressures, psi being atmospheric pressure, after each 4 hour period of sparking. It should be noted that the plug having the iridium insert and fabricated in accordance with our invention shows no substantial variation in sparking voltage over the 24 hour period of testing at each of the pressures indicated. This data is interpreted to mean that the iridium insert used in accordance with our invention results in a plug which is subject to very little wear under the accelerated life test conditions described. These conclusions are supported by a visual examination of the plugs which show that the iridium insert had no erosion at the same time that the Inconel shell tip to which the insert was welded showed a slight amount of erosion, such erosion being smooth and even.

From the foregoing description it is readily apparent that applicants have developed an igniter plug adapted for continuous sparking operation for extended periods of time at temperatures as high as about 2000 F. and under high vibratory conditions. Our invention is further described in the claims which follow.

What is claimed is:

1. In an igniter plug capable of continuous sparking operation for extended periods of time at elevated temperatures up to about 2,000 F. and including a hell body constituting the ground electrode, an insulator positioned within said shell body in gas-tight relationship therewith and having a centerbore therethrough, a center electrode positioned in said centerbore, the sparking action occurring across the spark gap between said ground and center electrodes and across the surface of said insulator, the improvement comprising a generally ring shaped insert formed from ductile iridium metal having high oxidation and spark erosion resistance at elevated temperatures, said insert being electron or laser beam welded to said shell and forming an integral part thereof on one side of said spark gap, the weld zone being narrow and well-alloyed with minimum recrystallization to form a strong bond under the high temperature, high vibration operating condi- 5 1% igniter plug as set forth in claim 1 wherein said insert has a fibrous structure for ductility and high resistance to fracturing.

3. An igniter plug as set forth in claim 2 wherein said weld zone penetrates the full depth of said insert to assure good heat transfer from said insert to said shell body. 

1. In an igniter plug capable of continuous sparking operation for extended periods of time at elevated temperatures up to about 2,000* F. and including a shell body constituting the ground electrode, an insulator positioned within said shell body in gastight relationship therewith and having a centerbore therethrough, a center electrode positioned in said centerbore, the sparking action occurring across the spark gap between said ground and center electrodes and across the surface of said insulator, the improvement comprising a generally ring shaped insert formed from ductile iridium metal having high oxidation and spark erosion resistance at elevated temperatures, said insert being electron or laser beam welded to said shell and forming an integral part thereof on one side of said spark gap, the weld zone being narrow and well-alloyed with minimum recrystallization to form a strong bond under the high temperature, high vibration operating conditions.
 2. An igniter plug as set forth in claim 1 wherein said insert has a fibrous structure for ductility and high resistance to fracturing.
 3. An igniter plug as set forth in claim 2 wherein said weld zone penetrates the full depth of said insert to assure good heat transfer from said insert to said shell body. 